<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1"><br>
<input type="text" id="i2"><br>
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)"><br>
<div>结果:</div>
<script>
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let d = document.querySelector("div");

    function f(x) {
        if(isNaN(i1.value)||isNaN(i2.value)){
            d.innerText = "输入错误!";
            return;
        }
        switch (x){
            case 1:
                d.innerText = "结果: " + (parseFloat(i1.value) + parseFloat(i2.value));
                break;
            case 2:
                d.innerText = "结果: " + (i1.value - i2.value);
                break;
            case 3:
                d.innerText = "结果: " + i1.value * i2.value;
                break;
            case 4:
                d.innerText = "结果: " + i1.value / i2.value;
                break;
        }
    }

</script>
</body>
</html>